Radio-frequency energy transfer systems are mostly based on inductive or far-field coupling. Both coupling techniques are sensitive to the orientation and misalignment of the receive antenna. In inductive coupling, circuit techniques have been invented to make the mutual coupling less sensitive to the displacement and tilting of the receive antenna. In far-field coupling, the use of circularly polarized antenna is a solution but the efficiency is reduced.
Multiple antennas have been used in wireless communications. Signals from multiple received antennas are coherently combined at radio frequency (RF). This reduces the noise power and hence increases the signal-to-noise ratio for better communication. However, this technique is not effective for energy transfer or harvesting because this reduces the receiving angle. There is a higher probability that the transmitter and the receiver are not aligned, and the energy transfer efficiency is degraded.